1. Technical Field of the Invention
The present invention relates to mobile radio telephone systems and, in particular, to a method and apparatus for identifying a call within a mobile telephone system.
2. Description of Related Art
When a mobile subscriber originates or receives a phone call, a mobile telephone switching office ("MTSO") (also referred to as a mobile switching center (MSC) or exchange) collects charging data during the call and outputs it in the form of a call billing record (a "toll ticketing record" or a TT record) at the end of the call. Such charging data may include, for example, a call identification number, the mobile station number, the subscriber number, the number called and the time duration of the call. These billing records are then stored on a recording medium such as a magnetic tape or a hard disk and are later processed to produce telephone bills that can be sent to the subscribers. The recorded charging data, such as the subscriber number, enable the billing center to retrieve and correlate all the charges belonging to a particular subscriber and produce a single telephone bill for that subscriber.
When two or more adjacent cellular radio coverage areas having different serving exchanges sharing a geographical boundary are provided, roaming functions within the cellular system allow calls to start in one exchange and terminate in another. Therefore, when a moving mobile station having a call in progress crosses the boundary between two adjacent systems, the call is "handed over" from the initial exchange ("anchor exchange") to the second exchange ("serving exchange"). Thus, calls are handed over not only from cell to cell during the normal course of operation within a cellular communication exchange but also from one exchange to another. The process of handing over calls from one exchange to another is called an interexchange handoff or an intersystem handoff.
If a cellular call is served entirely by the anchor exchange, all incurred fees are recorded by the anchor exchange and later outputted in the form of call billing records. However, if the mobile travels through and uses the cellular communication facilities of more than one MTSO, each MTSO produces a billing record to represent the service it provided to the roaming subscriber during the call. Because of the fact that no subscriber wants multiple bills for a single call, the billing system has to somehow correlate the services provided by the multiple exchanges and consolidate all the separate billing records from the multiple serving exchanges into a single bill. Conventionally, as soon as a serving exchange is no longer servicing the mobile station, either because that mobile station has traveled out of the current coverage area or has terminated the call, the serving exchange produces the necessary billing records and transports them back to the anchor exchange using the same telecommunications network used by the mobile station during the call. However, this puts additional traffic demand on the telecommunication network and creates undesirable system effects. Alternatively, each serving exchange could separately dump the billing records into a separate network or to a magnetic tape. But, because of a lack of common identifier, those billing records received from multiple serving exchanges cannot easily be correlated with each other and to a particular call the mobile subscriber has made utilizing those multiple serving exchanges. In other words, the conventional system does provide a mechanism for correlating different mobile calls to a particular subscriber but does not have a mechanism for correlating a number of different logical legs of a single mobile call.